A conventional communication signal processor receives a modulated signal (e.g., FM, PM, FSK) that is mixed and demodulated to produce an analog voltage signal. During demodulation, or immediately following it, a bandwidth limiting filter is applied. The filtered, analog voltage signal is then converted to digital form by integrating the signal and comparing it to a threshold voltage to produce a digital signal. Integrated signal voltages within selected voltage ranges are assigned corresponding digital magnitudes (e.g., binary magnitudes). The assignment of a digital magnitude is referred to as a bit or symbol decision.
In all electronic communications, it is desirable to transmit information on narrow bandwidths at low power. Frequency filtering narrows the bandwidth of a communication signal. In high capacity communications, many communication signals are carried within a given frequency allocation or channel. Increasing the degree of filtering of each signal on the channel (i.e., narrowing the bandwidth of each signal) increases the number of separate signals that the channel can carry (i.e., the channel capacity). In high capacity communications, including satellite communications, increases in the capacity of communication channels are extremely valuable. The problem is that increasing the degree of bandwidth narrowing by signal filtering decreases signal integrity due to inter-symbol interference. In inter-symbol interference, the signal that represents a digital symbol during a given symbol period is in effect contaminated with signal components from other symbol periods.
A multi-symbol analysis process of the present invention compensates for inter-symbol interference. The multi-symbol analysis process allows more narrow bandwidth digital filtering, which reduces the bandwidth required to correctly determine a digital symbol represented by an analog signal for a given signal to noise ratio. As a result, the multi-symbol analysis process can markedly increase the capacity of communications channels, including satellite communication channels.
The multi-symbol analysis process determines a digital value for a symbol represented by a selected symbol period by correlating a digitized sample within and outside the selected symbol period with a sum of weighted signal components. The sum of weighted signal components includes contributions from the selected symbol period and symbols from a number of other symbol periods that occur before and after the selected symbol period.
The symbol decision process also determines a quality factor of the symbol decision which may be used in error correction. When an error is detected in a group of symbols using, for example, conventional error detection techniques, the quality factor dictates the correction to the symbol. In a group of binary symbols, the quality factor can indicate the bit that is most likely erroneous and thus most eligible for correction. In multi-level symbols, the quality factor can likewise indicate which symbol is most eligible for correction and also the direction of correction, that is, whether the symbol value should be increased or decreased.
Additional objects and advantages of the present invention will be apparent from the detailed description of the preferred embodiment thereof, which proceeds with reference to the accompanying drawings.